Minimally invasive surgical techniques are known for performing medical procedures within the cardio-vascular system. Exemplary known procedures include the steps of passing a small diameter, highly-flexible catheter through one or more blood vessels and into the heart. When positioned as desired, additional features of the catheter are used, in conjunction with associated equipment, to perform all or a portion of a medical treatment, such as vessel occlusion, tissue biopsy, or tissue ablation, among others. Almost always, these procedures are performed while the heart is beating and blood is flowing. Not surprisingly, even though visualization and positioning aids are adequate for general placement of the device, maintaining the device in a selected position and orientation can be difficult as the tissue moves and blood flows, especially during a procedure that must be done relatively quickly.
One such minimally-invasive technique includes the use of catheter based devices, employing the flow of cryogenic working fluids therein, to selectively freeze, or “cold-treat”, targeted tissues within the body. Catheter based devices are desirable for various medical and surgical applications in that they are relatively non-invasive and allow for precise treatment of localized discrete tissues that are otherwise inaccessible. Catheters may be easily inserted and navigated through the blood vessels and arteries, allowing non-invasive access to areas of the body with relatively little trauma.
A cryogenic catheter-based ablation system uses the energy transfer derived from thermodynamic changes occurring in the flow of a cryogen therethrough to create a net transfer of heat flow from the target tissue to the device, typically achieved by cooling a portion of the device to very low temperature through conductive and convective heat transfer between the cryogen and target tissue. The quality and magnitude of heat transfer is regulated by the device configuration and control of the cryogen flow regime within the device.
A number of medical conditions may be treated using these ablative techniques or devices. For example, atrial fibrillation is a medical condition resulting from abnormal electrical activity within the heart. This abnormal activity may occur at regions of the heart including the sino-atrial (SA) node, the atrioventricular (AV) node, the bundle of His, or within other areas of cardiac tissue. Moreover, atrial fibrillation may be caused by abnormal activity within an isolated focal center within the heart. These foci can originate within a pulmonary vein, and particularly the superior pulmonary veins. Atrial fibrillation may be treatable by ablation of the abnormal tissue within the left atrium and/or the pulmonary vein. In particular, minimally invasive techniques, such as those described above, use ablation catheters to target the pulmonary vein in order to ablate any identified foci having abnormal electrical activity.
For atrial fibrillation, a cryogenic device is generally positioned at the ostium of a pulmonary vein (“PV”) such that any blood flow exiting the PV into the left atrium (“LA”) is completely blocked. At this position the cooling of the balloon system may be activated for a sufficient duration to create a desired lesion at the PV-LA junction. During the operation of a medical device, such as a cryogenic catheter, in a therapeutic procedure to treat a blood vessel, the heart or other body organ, its desirable to establish a stable and uniform contact between the thermally-transmissive (i.e., “cold”) region of the cryogenic device and the tissue to be treated (e.g., ablated). In those instances where the contact between the thermally-transmissive region of the cryogenic device and the tissue to be treated is non-uniform or instable, the resulting ablation or lesion may be less than optimal.
Difficulties arise in establishing or maintaining optimal positioning and contact between the treatment device and the target tissue. In particular, potential limitations of the cryogenic (or other thermal treatment) technique include the duration of time to create a transmural lesion, and the negative effects the high blood flow from the PV's has on the thermal efficacy and efficiency of the treatment. For example, the blood flow from the PV's may push the ablation device out of the PV since the blood flow is directed from the PV's into the LA in the opposite direction of the balloon placement, which is positioned at the ostium of the PV. Further, the blood contacting and/or flowing past the treatment device or ablation catheter has a temperature of 37° C., which raises the temperature of the treatment device, thereby reducing the thermal efficacy and efficiency of the device when attempting to achieve low temperatures rapidly.
In view of the above, it is desirable to provide for the improved isolation and corresponding treatment of tissue targeted for cryogenic or other thermal therapy to increase the thermal efficacy and thermal efficiency of the treatment.